Correlated photon pair emission is a prerequisite for the realization of entangled photon sources in various forms such as polarization entanglement, time-energy entanglement and time-bin entanglement, which form one of the key building blocks for applications in quantum information processing and computing [1], quantum communication [2], as well as imaging and sensing with resolutions exceeding the classical limit [3]. The generation of correlated photon pairs in various forms has been demonstrated through spontaneous parametric down-conversion (SPDC) in a diverse range of second-order nonlinear media [1a] and through spontaneous four wave-mixing (SFWM) within third-order nonlinear media [2a, 3a, 4a, 5a, 6a, 5, 7a].
To deliver the compactness, scalability and efficiency required by future optical quantum circuit devices, solutions focusing on an integrated on-chip approach have been recently studied and developed, including integrated quantum circuits, sources [5a, 6a, 5, 7a] and detectors [4]. The use of nonlinear micro cavities [5, 6] with narrow resonances and high Q-factors, i.e. below threshold pumped high-Q optical parametric oscillators, are of special interest since, in contrast to waveguides, such nonlinear micro cavities offer an enhancement in photon pair generation efficiency as well as a narrow photon pair bandwidth, rendering them compatible with quantum optical devices such as quantum memories for example. More importantly, resonant nonlinear cavities such as integrated ring resonators offer the possibility of generating correlated photon pairs on multiple signal/idler frequency channels [16] due to their periodic resonance structures. This multi-channel characteristic is beneficial for advances in quantum information processing, i.e. generating large quantum states for computation or realizing parallel operations.
Besides, the generation of quantum correlated and entangled photon pairs [16] through spontaneous four wave-mixing resonant nonlinear elements such as nonlinear microring resonators finds many applications in the generation of optical frequency combs [14, 15]. As the resonance bandwidths are very narrow, they are usually excited using a continuous wave (CW) laser with a spectral bandwidth smaller than that of the resonance [14-16].
Specifically, exciting a narrow resonance with an external laser is more efficient if a continuous wave (CW) laser is used, as the pump laser has a narrower spectral bandwidth than the resonance, therefore allowing high power transfer to the resonance [14-17]. However, with a continuous wave (CW) laser it is not possible to predict the time when photon pairs are generated, and defining an electronic system trigger for the synchronization with other components such as optical modulators is typically not possible. Pumping with a pulsed source is therefore desirable for many applications as it allows synchronizing the system to the repetition rate of the pump laser and thus to the generated photon pairs.
Furthermore, the optical quantum properties of the generated photon pairs rely on the pump configuration. If the resonator is pumped with a continuous wave (CW) laser, the generated photon pairs are not single-frequency mode and thus not pure [18] since the excitation bandwidth is not equal to the phase-matching bandwidth, leading to the often undesired generation of non-separable, i.e. frequency-entangled, states [15a] within a single resonance. Photons with high purity are generated only if the spectral bandwidth of the excitation field, in addition to being Fourier-limited, is perfectly matched to the bandwidth of the generated photons [15a], which can only be the case for a pulsed excitation.
Photons with high purity are generated only if the spectral bandwidth of the excitation field is perfectly matched to the bandwidth of the generated photons, which is the case with a pulsed excitation.
Exciting a narrow resonance efficiently with an external pulsed laser is very difficult to accomplish. A slight central frequency and/or bandwidth mismatch between the laser and the resonance deteriorates the coupling efficiency, with the result that most of the power is not coupled into the resonance and therefore unused and lost. In addition, the unused optical power counts towards the damage threshold of the device, often posing a limit to the available input power. Even more importantly, this type of excitation possesses inherent instabilities due to environmental or optically-induced thermal fluctuations, responsible for spectral shifts of resonance frequency and leading to detrimental effects in the photon pair generation rate, photon purity, etc. Furthermore, narrow spectral bandwidth pulsed lasers, i.e. in the 100 MHz range, are very difficult to realize, and, even if realized, using a narrow external laser moreover requires active locking of the laser frequency to the resonator in order to reach practical emission characteristics, which greatly increases complexity.